Bones Deforming Mesh that it's attached to
For some reason you've scaled your bone on its Y axis (Z global axis when the bone stands up) when you were in Pose mode. You need to select it in Pose mode and press altS to reset the scale. The parented mesh will come back to its normal proportion by the way...



If you want your bone to be longer than it is, you need to scale it in Edit mode, not Pose mode, which is made for animation. If you think that the Octahedral display mode is not convenient, you can switch to B-Bone mode for example, then you can change the bone thickness with 
ctrlaltS and drag.